1. Field of the Invention
The present invention relates to a TFT and an LCD using the same, and more particularly, to a TFT usable as a light sensor, a TFT substrate using the TFT, a method of fabricating the TFT substrate and an LCD.
2. Description of the Related Art
In general, flat panel displays are one type of display having a slim profile and providing a flat picture. Flat panel displays include liquid crystal displays (LCDs), which are widely used as notebook computer monitors, organic electroluminescent devices (OELDs), which are widely used in cellular phones, etc. In such flat panel displays, thin film transistors (TFTs) are generally used as switching elements for independently operating the respective pixels.
FIG. 1A is a plane view of a thin film transistor according to a related art, and FIG. 1B is a sectional view taken along the line I-I′ of FIG. 1A.
Referring to FIGS. 1A and 1B, the related art TFT includes a semiconductor layer 2 and a gate electrode 6 formed on a substrate 1. The semiconductor layer 2 is made of amorphous silicon or polycrystalline silicon (also referred to as polysilicon). A source region 3 and a drain region 4 are formed at selected portions of the semiconductor layer 2 such that the gate electrode 6 partially overlaps the source region 3 and the drain region 4. Between the semiconductor layer 2 and the gate electrode 6, a gate insulating film 5 is interposed. When an external voltage is applied to the gate electrode 6, a channel is formed in a channel region between the source region 3 and the drain region 4. Although not shown in the drawings, a metal electrode is formed on upper surfaces of the source region 3 and the drain region 4. By doing so, the TFT is electrically connected with an interconnection line for transmitting data or a pixel electrode disposed on each pixel region in an LCD to independently operate each pixel.
In a general flat panel display, a light sensor is used to sense the amount of light incident from an outside to adjust the brightness of a picture. A TFT can be used as such a light sensor. When light is incident into the light sensor TFT, light induced current is generated, even though the light sensor TFT is not turned on. The intensity of the light induced current is proportional to the amount of incident light. However, the related art TFT as shown in FIGS. 1A and 1B has the following drawback when it is used as a light sensor.
First, the TFT illustrated in FIGS. 1A and 1B has a reduced light-receiving area due to device structures positioned between the incident light and the source and drain regions. Again referring to FIG. 1A, light from an outside the TFT is incident on the source region 3 and the drain region 4, except for a portion of each shielded by the gate electrode 6. Additionally, since metal electrodes are formed on the source region 3 and the drain region 4, the light-receiving region of the light sensor TFT is substantially reduced. In addition to the above, the related art TFT has a drawback in that the light sensing capability is low, which will be described in more detail below.
In response to incident light, electron-hole pairs are generated in the source region 3 and the drain region 4. The generated pairs of electrons and holes move along the channel region, so that light induced current is generated. However, the electron-hole pairs generated in the source region 3 and the drain region 4 may not have a sufficient energy to pass through the channel region prior to recombination.